Electric discharge machines (EDM) are commonly used for making precise cuts, holes and otherwise shaping electrically conductive materials, such as steel, copper and the like. EDM machines employ a thin, consumable hollow wire, typically made of copper, using the spark discharge of the wire (internally cooled with water flow through it) to effect the EDM machining of various parts. In order to guide the wire, it is necessary to provide a device to center it and to hold the wire as it revolves and is moved downwardly through the EDM apparatus.
When EDM apparatus is used to bore holes or other shapes through a work piece, the hollow wire electrode is fed through the guide onto the work piece, where electro-erosion between the wire and the work piece forms a bore or hole through the work piece. This hole is a precise image of the wire electrode. In this type of machine, the wire electrode continuously is consumed during the cutting process; so that it continuously must be replenished by feeding it through the centering guide.
When an EDM apparatus is used to cut through a work piece in a precise pattern, a hollow wire electrode is unwound from a supply spool and is wound back around a return spool, with the spools both rotating to provide a precise constant rate of movement of the wire between them. Again, a centering wire guide is utilized between the two spools to accurately locate the moving wire electrode with respect to the workpiece. This causes the EDM discharge, which occurs between the wire and the work piece, to be accurately located with respect to the work piece which is moved under the control of a computer operated system to effect the desired cut through the work piece. In this type of system, the wire electrode is only partially consumed; but it still must be continually replenished as the exterior surface of the electrode is depleted through the discharge operation.
It has been common to use an orifice or circular collar wire guide of fixed diameter to guide the moving wire electrode in an EDM apparatus. Typically, the orifice or collar is manufactured to be slightly larger than the external diameter of the wire electrode. The United State patents to Iwasaki U.S. Pat. No. 5,086,202 and Fricke U.S. Pat. No. 5,214,260 both disclose fixed diameter wire guide in the form of circular collars. Apart from the absolute precision with which the orifices or circular openings in such fixed diameter wire guides must be manufactured, the guides also must be replaced when they become even slightly worn. In addition, if an EDM apparatus is changed over from the use of a machining wire of one diameter to a wire of another diameter, it is necessary to replace the wire guide in order to accommodate the different diameter wire electrode. Each time a new diameter is required, or replacement of an existing orifice because of wear is required, it is necessary to shut down the EDM apparatus while the changeover or replacement takes place. Such xe2x80x9cdown timexe2x80x9d is non-productive, and results in increased overall cost of EDM machining, which is not desirable.
The United States patent to Shin U.S. Pat. No. 5,958,260 is directed to a three point guide for centering the wire electrode of an EDM machine. This patent, however, discloses a system which requires fixed rod diameters selected for each different wire diameter to be used by the guide. As a consequence, the changeover requirements which are mentioned above, with respect to the Iwasaki and Fricke patents, also apply to the Shih patent apparatus requiring precision rod diameters and necessary down time for effecting replacement and changeover of the guide wire rods themselves.
Attempts to provide a more universal centering of the wire electrode of an EDM machine have included three point supports, or multi-point supports which are spring biased to hold the guides against the wire electrodes. The Unitde States patent to Nakayama U.S. Pat. No. 4,686,344 employs three balls or three rings which are biased by springs or fluid cylinders to engage the wire electrode, and thereby center it between the balls or rings during the feeding of the wire in the EDM machining operation. The use of spring force, however, as disclosed in the Nakayama patent, is not entirely reliable for accurate centering because the springs themselves may exhibit different amounts of force, which would then allow the wire electrode to deviate from its desired path.
Another spring operated centering device is disclosed in the Unitde States patent to Walser U.S. Pat. No. 4,791,264. This patent uses a wire guide with a pair of L-shaped legs, which are biased by means of a centering spring around the electrode wire, to provide the desired tension and centering of the wire. Once again, this is a spring biasing mechanism, subject to the inherent disadvantages of such mechanisms, which reduces its reliability for precision machining operations.
Two other United State patents Truty U.S. Pat. No. 5,585,013 and Aso U.S. Pat. No. 4,833,291, and are directed to wire guides providing a three point support. The device of Truty uses three or more spherical-balls which are biased against the wire by means of gravity as the balls rest on the walls of a generally tapered bore. There is little disclosure in the patent as to how the device disclosed in Truty actually works, although it appears that a spring bias is used to press the balls downwardly against the tapered surface, and therefore, toward the center against the wire electrode of the EDM apparatus.
In the device of the Aso patent, a three point support also is provided. A triangular groove is provided in one guide; and a flat surface is provided in another, to press against the wire, providing a three point support as the wire moves through the guide. The device of Aso relies upon spring tension to press the guide members toward one another against the sire passing through the guide.
Accordingly, it is desirable to provide an accurate, easily adjustable EDM machine wire guide which overcomes the disadvantages of the prior art mentioned above, which is easy to use, easy to adjust, and easy to maintain while providing precision accuracy in the apparatus in which is it used.
In accordance with a preferred embodiment of the invention, a support guide for a wire electrode in an electric discharge machine includes a main housing. This housing has spaced apart upper and lower portions, in the form of parallel plates with aligned openings through them, through which the wire electrode passes. Three alignment jaws each are pivotally mounted to the housing in the space between the upper and lower plates for rotation toward and away from one another, and from a central axis passing through the jaws and the openings in the upper and lower plates. A linking mechanism is pivotally attached to the jaws to move them simultaneously toward and away from the central axis by substantially identical amounts.
In a more specific embodiment of the invention, the alignment jaws include clamps each of which hold an elongated cylindrical ceramic rod, with the ceramic rods contacting the wire electrode during operation of the centering device.